Periodically, it is necessary to gain access to the inside of the water box compartments, for example to carry out checking and maintenance operations, and hence to be able to isolate these compartments from the primary circuit of the reactor when the latter is shut down.
Access to the compartments is gained by means of manholes which must be as small as possible and must also have a diameter which is just large enough for the service engineer to pass through without undue difficulty, thus making it possible to examine and service, if appropriate, the tubes of the tube plate.
Of course, during these operations, the primary fluid circulation circuit is shut down, in order to avoid the risk of hot or warm water entering the water boxes of the steam generator inadvertently. On the other hand, the orifice via which the primary fluid duct discharges has a fairly large diameter, and the risk of the operator or tools falling into the duct must be taken into account. Finally, this wide open orifice is phychologically disturbing, and for all these reasons it is essential to close this orifice as quickly as possible when an operator is to enter the interior of the bottom of the generator.
For this reason, as soon as the service engineer has entered the bottom of the steam generator he fits a shutter plate over the orifice of the primary fluid pipe. However, since the dimensions of this orifice are generally greater than those of the manhole, the plate must be introduced into the bottom in several pieces.
French Pat. FR-A-No. 2,340,489, owned by applicant, describes a shutting device comprising a cover made up of a central member and two C-shaped side members whose dimensions allow them to pass through the manhole. Each side member comprises, on either side of a scalloped-out section, two arms which are articulated at their ends to the corresponding arms of the other member about two hinges whose axes are aligned. The two side members are folded, one on top of the other, to pass through the manhole, and are then opened out to be applied to the bearing surface, each member covering a part of the said surface on either side of the pivoting axis of the hinges, and the central member covering the opening formed by the two scalloped-out sections side by side, and ensuring the rigidity of the assembly.
The main disadvantage of this type of shutter plate lies in the method by which it is fastened directly by means of screws. In fact, the latter are fitted into the periphery of the plate in a perpendicular direction to the latter and are then screwed into the circular carrier ring which is welded as close as possible to the tube connections. They are thus subjected to various mechanical stresses which arise, produced when the seal is fitted and the effect at the bottom by the pressure of the head of water present in the pipework during the shutdown of the reactor is taken up. These stresses are of a type which in the course of handling operations cause tensile failure of the screws to shock loads on the threads of the holes in the carrier ring, when then lead to long stays in the water box for the removal of the broken parts and for the necessary repairs. In addition, during this restoration the service personnel are subjected to the radioactive environment in the water box.
FR-A-No. 2,554,208 also discloses a shutter device which comprises at least two panels consisting of a light support coated with resin-bonded carbon fibers.
However, in this case it is not possible to make peripheral holes directly in the carbon fiber, as such machining would lead to a significant weakening of the mechanical strength of the shutting device. In addition, the solution which consists in reinforcing the areas around the holes by means of metal parts causes the same problems encountered in the device described above.